Conventional technologies of ECL based tunable lasers commonly implemented with mechanical actuators are still confronted with several technical challenges and difficulties due to the limitations of slower tuning speed, susceptibility to environmental vibrations and poor long-term stability/reliability. These shortcomings limit the applications of the ECL based tunable lasers to optical-fiber telecommunication systems. In the meantime, there is an ever-urgent demand to resolve these limitations and difficulties. Another technical difficulty faced by the tunable laser supplier is the requirement of a high side-mode-suppression-ratio (SMSR) that is demanded by the telecommunication industries for the purpose of providing high quality signal transmissions.
Specifically, in fiber telecommunications, tunable lasers are essential to provide system reconfiguration and reprogramming and the key parameters in optical networks are the speed, range, stability, and flexibility of wavelength tuning. Particularly, as digital video, audio and wide varieties of digital data and signals are transmitted via broadband networks, the lack of flexibility in network management becomes an ever-increasing headache for network managers among carriers. As optical network deployment approaches saturation and becomes ever so complicated and expensive, re-deployment and re-configuration become necessary. Therefore, flexible/tunable optical components become essential for next generation optical telecomm equipment. Different network systems implemented with tunable components are being developed currently or in the near future at major equipment makers. Furthermore, future applications may also require a laser with a higher power to compensate the components losses and a narrower line-width to battle with chromatic dispersion. An ECL based laser can potentially meet all these requirements.
Wada et al. disclose in U.S. Pat. No. 573,466 entitled “Method for Selecting Wavelength in Wavelength-tunable Lasers and Laser Oscillators Capable of Selecting Wavelengths in Wavelength-Tunable Lasers”, a laser resonator with a mirror on the input side and output side. A laser medium is placed in the resonator and can be oscillated in a predetermined range of wavelengths. A crystal to which is piezoelectric element is attached, the crystal receiving acoustic waves from the piezoelectric element in accordance with a desired wavelength; and a polarizing plate which is placed in the laser resonator and transmits only the output light beam having a prescribed plane of polarization or having a prescribed direction of light propagation among the output light beams from the laser medium. The apparatus thereby outputs only the desired wavelength. The tunable lasers disclosed by Wada et al. is limited by using a collinear configuration of the AO crystal, in which an acoustic wave and light wave propagate along the same direction that has shown very limited usefulness in the technology fields of telecommunication. The filtering line-width produced by such laser is very wide, much wider, e.g., an order of magnitude wider, than the channel spacing of 0.2 nm of a 25 Ghz spaced telecom tunable laser or 0.4 nm for a 50 Ghz spaced telecom tunable laser. And the size of the crystal would likely be an order of magnitude longer then a size that would fit into a miniaturized tunable laser package suitable to be implemented in the modern telecommunication systems. Furthermore, Wada's tunable laser is still limited by the inherent characteristics of an Acousto-optical filter that there will be wavelength shifts introduced by the acoustic wave. If a backward path is applied, there would be double shifts and the tunable laser as disclosed by Wada et al. would not be able to sustain a viable oscillation suitable for application in modern telecommunication systems.
Gutin disclosed in a Patent Application 20020018496, entitled “Tunable Diode Laser System Apparatus and Method”, a tunable laser system wherein the tuning of the laser is accomplished by a micro-mirror array (MMA) formed as two-dimensional linear array of micro-mirrors manufactured by applying the micro-electromechanical system (MEMS) technologies. Due to the limitation of the speed of movements of the micro-mirrors, the tuning speed of the diode laser system is limited to a microsecond range, in actuality, with electronic servos and complicated servo and tuning algorithms, the tuning speed can hardly reach below the millisecond barrier as most of the MEMS based switching components have demonstrated. Furthermore, potential problems associated with moving parts, such as problems of long-term reliability and stability, also hinder practical applications of the diode lasers as disclosed by Gutin in the fiber network systems that demand a high level of reliability and stability over long term operations.
Another difficulty of the ECL based tunable lasers is a critical problem with mode hopping and it often limits the manufacturability in terms of cost and yield, and in most cases, becomes its limiting factor for optical and tuning performances. It is much more difficult to make an ECL-based laser provided with an operational feature to tune continuously across the entire optical telecommunications bands. It is often discretely tunable only on the ITU grids. The mechanism of mode hoping can be understood with a phenomenon that the selected laser cavity mode corresponding to the longitudinal oscillation wavelength and the narrow-band filter mode corresponding to a center wavelength do not align with each other. The mode hopping problem results in a competition of adjacent laser cavity modes, both allowable within the narrow-band filter line-width that leads to the laser cavity to be in an unstable state thus causing an alternating oscillation. Such a problem is unavoidable if in a tunable ECL laser, the only tuning mechanism is the tuning of filter center frequency, as in the cited configurations of the referenced patents. In most ECL-based tunable lasers, often more then one tuning mechanisms are implemented where the motions are coordinated for continuous tuning without mode hope. That is the reason why it becomes very difficult and costly to provide an ECL-based tunable laser with continuously tunable capability over the entire optical band.
For these reasons, an ECL based laser when limited by the technical difficulties even with several performance advantages, the conventional ECL-based lasers are still of limited usefulness for application to the fiber networks. Therefore, a need still exists in the art of optical fiber system and component manufacturing and design, particularly those related to ECL-based lasers, to provide new and improved system and component configurations and designs to overcome the above-mentioned technical difficulties and limitations.